Field of Invention
The present disclosure pertains to audio equalization.
Background of the Invention
An audio signal is a representation of sound as an electrical voltage waveform. Audio signals which correspond to audible sounds have frequencies between approximately 20 and 20,000 Hz. Audio signals may be characterized by parameters, like, for instance: bandwidth, power level, and voltage level. In many situations, a signal needs to be processed in order to modify its character and, correspondingly, the sound underlying the signal.
Equalization is one process by which the character of an audio signal can be changed. Bell or peak filters are widely used in equalization. Equalizers are the circuits or equipment used to achieve equalization.
In equalization, an audio signal's character is changed by varying the center frequency, the Q-factor (or bandwidth), and gain (decibel) of the frequency response curve of an audio system. Known parametric equalizers are configured to manipulate these three variables independently. The change in character of an audio signal can be quantitatively represented by a frequency response bell curve, which represents differences between input signals and output signals as a plot of amplitude versus frequency. In the bell curve, the gain is the height of the curve, the frequency is the center of the peak, and the Q-factor is the width of the curve.
A peak filter is a resonant filter, meaning that the frequencies that are affected by the filter will “resonate.” It translates into the time domain by producing an oscillating, or resonant, impulse response, which characteristics depends on the parameters of the peak filter.
One of the problems of audio equalization is the management of resonances, which can occur at the center frequency of a bell shaped equalization filter. A resonance in equalization can be described as an amplitude peak of the equalization curve. Generally, a resonance can relate to the amplitude of the sound being louder at a specific frequency (resonant frequency) than the rest of the audio in the same sound.
A standard peak filter has a resonance which is defined primarily by the gain and the Q-factor. The steeper (the higher the Q-Factor) the peak filter is, then the more resonant the filter is. A steeper filter exhibits a longer impulse response. A wider filter exhibits a shorter impulse response, but with a higher amplitude, and can be described as less resonant. So the more resonant the filter, the longer the impulse response.
Independent manipulation of the variables (frequency, gain, and Q-factor) results in limited equalization ability. For instance, manipulating the gain fixes the relationship between frequency and amplitude of the frequency response curve. This means that equalization has been limited to adjustments in the scaling of a fixed frequency response curve. In view of the foregoing, a need exists for equalization of an audio signal that does not result in scaled frequency response curves.